We describe the identification and characterization of a transcription factor encoded by the atf1+ gene of the fission yeast Schizosaccharomyces pombe. The factor Atf1, contains a bZIP domain at its C‐terminus with strong homology to members of the ATF/CREB family of mammalian factors and in vitro binds specifically to ATF/CRE recognition sites. Furthermore the ATF‐like binding activity detected in extracts from fission yeast cells is entirely lost upon deletion of the atf1+ gene. Upon growth to saturation, fission yeast cells exit the mitotic cycle and enter a G0‐like stationary phase. However, on rich medium, entry of atf1‐ cells into stationary phase is restricted and they rapidly lose viability; this does not occur on minimal medium unless cAMP levels are raised. Thus stationary phase entry appears to be regulated negatively by cAMP and positively by Atf1. atf1‐ cells are also sterile and this sterility appears to be due to a combination of two defects: first, upon nitrogen starvation the majority of atf1‐ cells fail to arrest in the G1 phase of the cell cycle and second, the induction of ste11+ expression is lost. Thus expression of ste11+ represents a second example of an event that is negatively regulated by the cAMP pathway and positively regulated by Atf1. Despite their close association however, these two regulatory pathways function independently and Atf1 activity is not directly modulated by cAMP levels or mutations that alter the activity of components of the cAMP signalling pathway. Thus Atf1 is a transcription factor that plays an important role in the response of cells to adverse environmental conditions, which is to exit the mitotic cell cycle and either sexually differentiate or enter a resting state.

In fission yeast, maintenance of genome ploidy is controlled by at least two mechanisms. One operates through the Cdc2/Cdcl3 kinase, which also involves the CDK inhibitor Ruml, and the other through the S-phase regulator Cdcl8. By screening for sterile mutants that show increased ploidy, we have identified a new gene, popl ÷, in mutants that become polyploid. The popl mutation shows a synthetic lethal interaction with the temperature-sensitive cdc2 or cdcl3 mutation. In a popl mutant Ruml and Cdcl8 proteins become accumulated to high levels. The high ploidy phenotype in the popl mutant is dependent on the presence of the rural ÷ gene, whereas the accumulation of Cdcl8 is independent of Ruml. The predicted sequence of the Popl protein indicates that it belongs to a WD-repeat family with highest homology to budding yeast Cdc4, which participates in the ubiquitin-dependent pathway. Consistent with this notion, in a mutant of the 26S proteasome, higher molecular weight forms of Ruml and Cdcl8 are accumulated corresponding to polyubiquitination of these proteins. In the popl mutant, however, no ubiquitinated forms of these proteins are detected. Finally we show that Popl binds Cdcl8 in vivo. We propose that Popl functions as a recognition factor for Ruml and Cdcl8, which are subsequently ubiquitinated and targeted to the 26S proteasome for degradation.

SME1 was cloned due to its high copy number effect: it enabled MATa/MAT alpha diploid cells to undergo meiosis and sporulation in a vegetative medium. Disruption of SME1 resulted in a recessive Spo- phenotype. These results suggest that SME1 is a positive regulator for meiosis. DNA sequencing analysis revealed an open reading frame of 645 amino acids. An amino terminal peptide of ca 400 amino acids in the deduced protein was similar to known protein kinases. Transcription of SME1 was regulated negatively by nitrogen and glucose and positively by MATa/MAT alpha and IME1, another positive regulator gene of meiosis. By complementation analysis, SME1 was found to be identical to IME2, which had been shown to be important in meiosis. These results suggest that IME1 product stimulates meiosis by activating transcription of SME1 (IME2) and that protein phosphorylation is required for initiation of meiosis.

Background: In the ubiquitin-dependent proteolysis pathway, a ubiquitin ligase (E3) is responsible for substrate selectivity and timing of degradation. A novel E3, SCF (Skp1-Cullin-1/Cdc53-F-box) plays a pivotal role in cell cycle progression. In fission yeast, F-box/WD-repeat protein Pop1 regulates the level of the CDK (cyclin-dependent kinase) inhibitor Rum1 and the S phase regulator Cdc18.

Many eukaryotic cells arrest the cell cycle at G 1 phase upon nutrient deprivation. In fission yeast, during nitrogen starvation, cells divide twice and arrest at G 1 . We have isolated a novel type of sterile mutant, which undergoes one additional S phase upon starvation and, as a result, arrests at G 2 . Three loci (apc10, ste9/ srw1 and rum1) were identified. The apc10 mutants, previously unidentified, show, in addition to sterility, temperature-sensitive growth with defects in chromosome segregation. apc10 ⍣ is essential for viability, encodes a conserved protein (a homologue of budding yeast Apc10/Doc1) and is required for ubiquitination and degradation of mitotic B-type cyclins. Apc10 does not co-sediment with the 20S APC-cyclosome, a ubiquitin ligase for B-type cyclins, and in the apc10 mutant the 20S complex is intact, suggesting that it is a novel regulator for this complex. A subpopulation of Apc10 does co-immunoprecipitate with the anaphase-promoting complex (APC). A second gene, ste9 ⍣ /srw1 ⍣ , encodes a member of the fizzy-related family, also regulators of the APC. Finally, Rum1 is a cyclindependent kinase (CDK) inhibitor which exists only in G 1 . The results suggest that dual downregulation of CDK, one via the APC and the other via the CDK inhibitor, is a universal mechanism that is used to arrest cell cycle progression at G 1 .